The mysterious electrical circuit powering the Northern lights has been revealed for the first time by four satellites flying in formation through the upper reaches of an aurora.
Researchers already knew that the colourful curtains of light are caused by high energy electrons zipping into the Earth鈥檚 atmosphere in high-latitude night skies.
But they have always been puzzled by calculations that show the electrons鈥� temperature must be several million degrees Celsius. The temperature of the solar wind, which blows electrons from the Sun out into the inner Solar System, is about a thousand times cooler than that.
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Now data from the European Space Agency鈥檚 four Cluster satellites, launched in 2000, shows electrons draining out of the ionosphere 鈥� the part of the atmosphere where molecules get ionised by solar radiation 鈥� into outer space. The data was analysed by Goran Marklund and colleagues at the Royal Institute of Technology in Stockholm, Sweden.
Patrick Newell of John Hopkins University in Laurel, Maryland says once the ionosphere is drained, huge electric fields can build. These catapult the solar wind electrons that enter the Earth鈥檚 magnetic sphere of influence down toward Earth, causing a light show in the upper atmosphere.
鈥淭his solves the puzzle of how the electric fields are sustained,鈥� he says. The acceleration of the electrons by the fields explains why their temperature rises.
Wax and wane
In January 2001, Cluster鈥檚 satellites passed 20,000 kilometres above a northern aurora as it waxed and waned at an altitude of 200 km. Although previous satellites have measured electric field and current flow during aurora, the fact that they were flying solo made it difficult to reconstruct the evolution over time of electric fields at fixed points in and above the atmosphere.
鈥淲hen you鈥檙e driving, you can鈥檛 tell if the rain stopped or you simply drove out of it,鈥� explains Newell.
Marklund presented his work on Tuesday at the American Geophysical Union meeting in San Francisco, California.
Journal reference: Nature (vol 414, p 724)